when a lord anwser you "on a more private date" that mean there is other lords nearby (on general map or on local map, such in castles, towns), so you have to speak to him when no one is nearby.
As for the anwser "i'm aint a plotter", i think its is related to the lord's personality (some are chivalerous, other are brutal, ect, ect) and your persuasion, honor, controversy and renown, and how much they like/dislike you. There might be other factors, such as the size of your army, but i lack real infos on that kind of lord's anwsere.